The invention is particularly suited for use in producing rock drilling bits of the type generally referred to as percussion drilling bits and will be described with reference thereto; however, as will become apparent, the invention could equally well be used to produce roller cone bits, polycrystalline diamond compact (PCD) bits, and similar bits of the type wherein the cutting is performed by hard material inserts carried in a drill body.
A conventional percussion drill bit comprises a steel drill body having a generally cylindrical mounting shark carrying an axially aligned cylindrical head defining a cutting face. A multiplicity of cylindrical, hard material cutting inserts, generally formed of sintered tungsten carbide, press-fitted in precision drilled openings in the cutting face. The exposed ends of the cutting inserts perform the actual rock cutting by abrading or crushing the rock into rock dust and small particles. The dust and particles are flushed from the drill hole by compressed air or other pressurized fluid supplied through a central passageway in the drill bit and out branch passageways opening on the cutting face. FIG. 1 is a depiction of such a conventional percussion type down-the-hole ("DTH") drill bit. Specifically, FIG. 1 shows a conventional DTH drill bit (10) comprising a drill body (20) having a connecting section (30), defining a rotational axis, means for coupling (35) the drill bit (10) to a percussive unit or other drill device (not shown), a cutting face (40) having a plurality of holes (42) therein (not shown), the cutting face being rigid with the connecting section, a central passageway (45) therethrough (not shown) with at least one branch passageway (50) extending from the central passageway and opening onto the cutting face and at least one recess (60); a plurality of cutting inserts (70) affixed to the cutting face in the plurality of holes, including gauge row inserts (72) located along the outermost periphery of the cutting face, embedded in the cutting face, each cutting insert comprising a carbide body having a rear mounting portion (not shown) embedded in the drill body and a cutting end portion protruding from the drill body.
Typically, the life of a rock drill bit is dependent on the life of the hard material cutting inserts. However, in certain rock formations, such as soft, fractured formations, the bit body itself is subjected to significant erosive wear. In cutting operations in such rock formations, failure of the drill bit often occurs prematurely because of erosion of the drill body, particularly in the area surrounding the cutting inserts. This erosion results in the weakening of the drill body in general. This erosion also may result in a loss of support for the cutting inserts. Specifically, erosion of the drill body at the site of fixation of the cutting inserts weakens the bond between the drill body and the cutting inserts. Eventually, erosion at the site of fixation results in the separation of the cutting insert from the drill body. Once one or more cutting inserts are separated from the drill body, the cutting inserts must be reaffixed to the drill. If reaffixation is not possibly, the drill bit must be retired. The separation of cutting inserts from the drill body is a particular concern with respect to the outer or "gage" row of inserts located along the periphery of the cutting face because the outer periphery of the cutting face is subjected to more erosive conditions than the rest of the cutting face.
In an effort to increase the durability of rock drill bits and to overcome the cutting insert separation problems noted above, various methods have been employed such as increasing the hardness of conventional steel bit bodies by using a higher carbon content steel and heat treating for high hardness; forming the drill bodies from a low carbon content steel and subsequently carburizing and case hardening the drill body; and carburizing and case hardening the cutting face while selectively preventing the penetration of carbon into the cutting face in the areas where the cutting inserts are to be affixed. Notwithstanding, materials of construction and designs which result in rock drill bits which can drill faster and last longer are constantly being sought.